7 The Discovery of Sperm in Higher Eukaryotes Semen has long been recognized as necessary for producing offspring. It is liquid, somewhat viscous, and usually clear or slightly cloudy in appearance; certainly the unaided eye can see no visible body within it. The Greeks, especially through Hippocrates and later Galen, embraced a theory of vital fluids, which they called humors. Blood was considered the major constituent of life, at least among vertebrates. It was considered the progenitor of semen in the male body, and believed to be the hereditary material that allowed a species to generate offspring in its likeness. Semen was endowed with a capacity to impose form on the pliable materialsuppliedbyfemales.Thatmaterialwasalsothoughttobeblood: sometimesitwasassociatedwithmenstrualblood,andsometimesitwas thought to be another type of semen. Female semen was not clarified, like male semen, but still bloodlike and clotted—a type of miniscule clay ready to be molded into shape by the empowering effect of male semen. For more than two thousand years, arguments were made about therelativerolesthatmalesandfemalesplayinforminganewindividual through their fluids, which were commingled after copulation. There were inside–outside theories in which the male supplied the outer components of the new baby. There were theories in which the female role was passive, being shaped exclusively by the male, forcing some observable phenomena, such as the equal contributions made to the skin color of the offspring of a black person and a white person, to be swept under a mental rug. Anthony van Leeuenhoek was the first to recognize that semen was not exclusively a liquid.1 In one of his letters to the Royal Society, he The Discovery of Sperm in Higher Eukaryotes 43 described “animalcules” present in a sample of semen. He did not use the term cells that Robert Hooke had introduced a few years earlier, in 1665. Hooke’s cells were found in the bark of cork trees. They were empty, rigid, and formed a buoyant honeycomb. Leeuenhoek’s animalcules were alive, swimming, vibrating, and had tails. They were like the animalcules he had earlier described in a variety of samples of water and fluidsheputunderhissinglelensmicroscope.Todaymostofthesewould be identified as “infusorians,” mostly protozoa and some of the more obscure microscopic phyla like rotifers that are multicellular. Leeuenhoek later noted that sperm of different animals had different morphologies. He suspected they played a role in the process of embryo formation, but he rejected the view that either the sperm or the egg alone led to the complete embryo.2 Over the next two centuries, there were many questions about sperm. If the sperm was indeed the progenitor of new offspring of copulating parents, why were there so many present in an ejaculate? Why would the Creator waste so many lives to produce one? If development was epigenetic, as Harvey had confirmed, repeating Aristotle’s day-byday examination of chick embryos, what possible role could an individual sperm play? Many biologists looked at the presence of sperm in semen from a different perspective. They thought sperm were parasites, not very different from the animalcules that Leeuenhoek encountered in his many samples of fluids from his own body and from other environments. As early as 1839, Martin Barry (1802–1865) disputed this parasite theory. He claimed sperm entered the rabbit eggs he observed. In 1841, Albert von Kölliker (1817–1905) compared spermatozoa in semen to red blood corpuscles. He concluded that they were an essential component of semen , but he did not agree with Barry that penetration of the egg was involved. He claimed such penetration would support a vitalistic interpretation of reproduction, akin to Harvey’s or Aristotle’s epigenetic theory of development.3 Theodor L. W. Bischoff believed that spermatozoa used their tails to agitate the egg, mixing the seminiferous fluid with the egg surface. He also thought that the sperm never entered the egg during fertilization; that the process was not mechanical but chemical.4 Much more convincing were the observations and experiments of the French diplomat 44 turned algologist, Gustav Thuret (1817–1875). In 1844, he studied the seaweed fucus and noted it produced two types of cells: an “oval spore” and an “antherozoid.” The antherozoids each had two cilia and were motile. In 1853, he placed antherozoids on the oval spores and found they produced new algal plants, but oval spores without such contact withantherozoidsdidnotproducealgalplants.Hereferredtohisfinding as evidence that the lower cryptogamic plants had “true sexuality.”5 In 1851,PrussianbotanistNathanaelPringsheim(1823–1894)wasthefirstto describe the process of fertilization. He used fucus and vaucheria for his studies, showing...

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